In the test equipment area, the availability of intelligent, low-cost digital logic circuitry has permitted many test functions, normally performed in the analog domain, to be performed instead in the digital domain. While there have been numerous efforts to shift more of such processing into the digital domain, the prior art to date has failed to provide an economic and compact digital signal processing unit which is capable of handling analog signals having a wide frequency range as well as a wide dynamic range.
Typical prior art devices either suffer from a limited frequency range, such as digital voltmeters (DVM), or are unable to accomodate low level analog signals in the presence of high level digital signals.
It is particularly important in certain applications to provide digital signal processing apparatus which can be isolated from the test subject matter as well as from the apparatus power source and result analyzing circuitry.
Further, where low level signals are being processed or provided to a circuit under test, it is a difficult task to maintain the quality of such low level signals and to also perform high speed digital processing in close proximity thereto.
There is an inherent difference in handling analog signals versus digitial signals. While each approach has advantages over the other, the techniques which provide such advantages in one approach do not easily coexist in the same environment with the techniques of the other. For example, in the analog area much effort is placed in maintaining a good signal to noise ratio and wide bandwidth as the signal is being handled.
In contrast, once a signal is digitized, bandwidth and signal to noise ratios are not a problem. In digital signal processing it is important to have a sufficient number of samples of a particular signal such that the particular signal can be reconstructed from the finite number of samples obtained. This often requires the use of high speed processing logic operating at signal levels much higher than the levels of the analog signals being processed. From the standpoint of a systems designer, the coexistence of low level, high frequency analog and high speed digital processing circuitry on the same board present difficult design problems.
A further difficulty with previous digital signal processing apparatus, specifically frequency measuring apparatus, is that the measurement methods employed often resulted in unacceptable error. More often than not, the frequency measured was substantially on the high side of the true frequency of the signal being measured. Often, such error was due to the inability of the apparatus to reject noise present on the waveform to be measured.